This invention relates to nanoscale infrared spectroscopy using Atomic Force Microscope based techniques and particularly to isolating tip dependent signals from background signals.
In recent years atomic force microscopy has intersected with infrared spectroscopy to provide spectroscopic characterization of materials with sub-micron spatial resolution. One commonly used technique is called Photo-Thermal Induced Resonance, as described by Dazzi et al in U.S. Pat. Nos. 8,001,830 and 8,402,819 and related applications, each incorporated by reference. In this technique a pulsed, tunable infrared source is used to illuminate a region of a sample. When the source is tuned to a wavelength corresponding to an absorption of the sample, a portion of the incident radiation is absorbed by the sample, rapidly heating the absorbing region. The rapid temperature rise creates a corresponding thermal expansion shock wave that produces a transient force on the tip of an AFM cantilever probe. The AFM cantilever then rings at one or more frequencies, corresponding to the contact resonance modes of the AFM cantilever. By measuring the amplitude of the cantilever response as a function of illumination wavelength, it is possible to create an absorption spectrum of regions of the sample.
In certain implementations of this technique, see FIG. 1, the sample 104 is mounted to a prism 106 which is illuminated by IR radiation 108, creating absorption induced oscillation of cantilever 100 due to sample expansion at tip 102. For this case, the prism 106 is chosen such that radiation 108 is totally contained within the prism 106 and sample 104 by total internal reflection. Thus there is essentially no radiation on the tip 102 and cantilever 100.
The implementation of FIG. 1 is advantageous in that it restricts the illumination to just the sample 104. However not all samples of interest are small enough or of the proper configuration or transparency to be mountable on a prism and illuminated from the bottom. An alternative, more generally applicable implementation is shown in FIG. 2. In this case, sample 204 is illuminated from the top by IR radiation 208. It should be noted that the figure is schematic and not drawn to scale. Tip apex 203 is very small compared to the minimum spot size achievable for the IR illumination. Thus illumination 208 illuminates the sample 204 in the region of the tip apex 203, but also illuminates the sample (and/or sample mount) away from the tip apex as well as the tip shank 202 and a portion of the cantilever 200. These other areas illuminated can affect the spatial resolution which can be compromised by at least two background sources in particular: (1) absorption of light by the cantilever and/or tip away from the tip apex; and (2) absorption of light by the sample, but in an area away from the tip apex. Both of these responses add to the total cantilever response. The resulting measured absorption spectra can then be heavily contaminated by these background signals, often obscuring the signal from the much smaller volume of sample material under the tip apex 203.
There is a large class of samples, which we will define as “in situ” samples, where bottoms up illumination is not suitable. These in situ samples are generally better measured in top-side illumination. For these samples the background problem is especially significant. In situ samples are samples that by their nature generally cannot be prepared for placement on an infrared transparent prism for bottoms up illumination. These include samples that are opaque over a wavelength range of interest and thus cannot be used for total internal reflection illumination from below. In situ samples also include samples where the regions of interest are on a predefined substrate, for example a defect or a thin film on a semiconductor wafer. Other examples include geological/petrochemical samples, wear tracks, substrates and devices used in data storage, coatings and deposited thin films, and similar samples. In situ samples can also include samples that cannot be readily prepared into thin sections on an infrared transparent prism, for example samples that cannot be readily microtomed, drop cast or spun cast. These can be samples that may be too hard and/or fragile to cut into thin sections (<1 μm thickness). Other examples can include pharmaceutical samples and powders that tend to crumble when cut. For this family of in-situ samples that cannot be readily measured in bottoms up illumination, top side illumination is highly desirable. But with topside illumination, the background absorption from the light incident on the cantilever and/or tip signals can significantly undermine the quality of the measured absorption spectra and can also significantly degrade the spatial resolution of the measurement.
An example of this excessive background is shown in FIG. 3. This figure shows two AFM-IR spectra obtained on a sample of poly-methylmethacrylate (PMMA). The dashed curve is taken in the bottoms up illumination scheme of FIG. 1 and fairly accurately reflects the absorption properties of the PMMA material. The solid curve shows a measurement obtained on the same sample, but using top side illumination. The spectrum clearly shows a significant increase in background absorption, obscuring much of the detail of the actual absorption properties of the PMMA material. The current invention details techniques for substantially reducing this background effect to allow more accurate nanoscale spectroscopic measurements, even with top-side illumination. A significant problem is that for unknown samples, which is of course what the PTIR technique is intended to identify, sometimes the background contribution to the spectra is larger than the signal from the tip signal, so it can be difficult to accurately measure the tip signal, and thus the true sample absorption spectra.